The key feature is that the error message triggered by the undefined expression
sizeof(STATIC_ASSERTION_FAILURE<0>), tends
to be consistent across a wide variety of compilers. The rest of the machinery
of BOOST_STATIC_ASSERT is just
a way to feed the sizeof expression
into a typedef. The use of a macro
here is somewhat ugly; however boost members have spent considerable effort
trying to invent a static assert that avoided macros, all to no avail. The
general conclusion was that the good of a static assert working at namespace,
function, and class scope outweighed the ugliness of a macro.